The use of MR imaging to record motion has long been of interest. Conventional techniques for collecting data for motion study required precise monitoring of the motion in addition to retrospective sorting and/or interpolating of data. This conventional cine magnetic resonance imaging required long imaging times and posed various technical difficulties.
It has now been discovered that image artifacts produced by movement of the object of interest during the acquisition of NMR data may be useful in motion imaging. These artifacts are commonly called "ghosts" and collectively comprise a ghost mask. In the image, ghosts appear as equally spaced replicas of the moving object in the phase encoding direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the ghosting is due to the inconsistent appearance of the object from view to view. The primary source of such ghosts is patient motion due to respiration or the cardiac cycle.
Ghosting has been considered problematic due to the potential for diagnostic misinterpretation when the ghosts are superimposed on the anatomical image. Previously, the attempt has been made to eliminate the ghost by methods such as gated NMR scanning. Recently, however a method for cancellation of image artifacts has been developed as described in U.S. patent application Ser. No. 718,678 to Xiang et al, filed Jun. 21, 1991 and incorporated herein by reference. The method employs a decomposition procedure to separate the ghost mask from the desired image. The ghost free image obtained is the averaged spin-density distribution during the entire course of the scan.
The dynamic image reconstruction method of the present invention is based on the ghost analysis disclosed in U.S. application Ser. No. 718,678. The dynamic imaging method disclosed herein avoids the need for monitoring motion and offers a higher data acquisition efficiency than conventional cine magnetic resonance imaging.